In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Network multiple-input and multiple-output (MIMO) and collaborative MIMO have been proposed for LTE. Factors to consider for MIMO include: geographical separation of antennas, selected coordinated multi-point processing approach (e.g., coherent or non-coherent), and coordinated zone definition (e.g., cell-centric or user-centric). Depending on whether the same data to a UE is shared at different cell sites, collaborative MIMO includes single-cell antenna processing with multi-cell coordination, or multi-cell antenna processing.
In the 3rd Generation Partnership Project (3GPP), coordinated multi-point transmission/reception (CoMP) is considered a tool to improve coverage, cell-edge throughput, and/or system efficiency. In coordinated multi-point transmission/reception (CoMP), transmission to a user is collaborated by multiple base stations or network points, acting together to remove interference. In some respects, CoMP focuses on user equipment (UE) located in a cell-edge region. In the cell-edge region, the UE may be able to receive signals from multiple cell sites and its transmissions may be received at multiple cell sites, regardless of the system load. However, if the radio signal transmissions are from multiple cell sites are coordinated, the downlink (DL) performance can be increased significantly. This coordination can vary in complexity from techniques that focus on interference avoidance to those which are more complex, e.g., where the same data is transmitted from multiple cell sites.
In terms of downlink CoMP, two different approaches are under consideration. A first approach is coordinated scheduling CS, which includes coordinated beamforming (CB). A second approach is joint processing JP, which includes joint transmission (JT). Beamforming is a signal processing technology that is used to direct the reception or transmission (the signal energy) on an antenna array in a chosen angular direction. Beamforming concentrates the array to energy coming from only one particular direction to apprehend signals in one direction and ignore signals in other directions.
Thus, including Single Point Transmission, there are in general modes of data transmission: the Single Point Transmission Mode (SP); the CoMP CS Mode; and the CoMP JP Mode. The single point transmission mode (SP) is a traditional transmission mode without CoMP. In general, CoMP CS is a transmission mode in which there is only a single user served by the coordinated nodes (e.g., eNBs), and which benefits essentially only the cell edge users. CoMP JP is a transmission mode in which there are multiple users served by the coordinated nodes (e.g., eNBs). Unlike CoMP CS, CoMP JP may benefit both cell edge users as well as cell central users.
In the coordinated scheduling CS approach, the transmission to a single UE is transmitted from a serving cell, exactly as in the case of non-CoMP transmission. However, the scheduling of transmission on the downlink from the serving cell to the UE, including any beamforming functionality, is dynamically coordinated between the cells (e.g., scheduled to take into consideration the transmissions from other cells) in order to control and/or reduce the interference between different transmissions. In principle, the best served set of users will be selected so that the transmitter beams are constructed to reduce the interference to other neighboring users, while increasing the served user's signal strength.
In the joint processing JP approach, the transmission to a single UE is simultaneously transmitted from multiple transmission points, across cell sites. The multi-point transmissions are coordinated as a single transmitter with antennas that are geographically separated. The joint processing JP scheme has the potential for higher performance, compared to coordination only in the scheduling. However, joint processing JP typically has more stringent requirements for the backhaul communications (e.g., more stringent requirements for intermediate links between the core network, or backbone, of the network and the small sub-networks at the “edge” of the entire hierarchical network communications).
While CoMP JP may benefit more users, from a single user's point of view, CoMP JP has a weaker performance than CoMP CS because of co-channel interference, especially when the channel matrices of the active users are not orthogonal enough, or the precoding matrix indices (PMIs) of the active users are not orthogonal enough. In the CoMP JP mode, as there are so many users who should be jointly processed by multiple coordinated eNBs including cell edge and cell central users, it is very complex to perform joint signal processing and associated scheduling. Moreover, CoMP JP mode needs a very high speed low delay backhaul link.
As discussed above, there are drawbacks applying CoMP CS or CoMP JP separately. What is needed is a method which benefits from aspects of both approaches.